1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a control rod spider assembly incorporating secure attachment joints for fastening the control rods to the spider structure.
2. Related Art
In a typical nuclear reactor, such as a pressurized water reactor (PWR) type, the reactor core includes a multiplicity of fuel assemblies. Each fuel assembly is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between and attached at opposite ends to the nozzles. A plurality of transverse support grids are axially spaced along and attached to the guide thimbles. A plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles are supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and out of the guide thimbles.
One common arrangement utilizing control rods in association with a fuel assembly can be seen in U.S. Pat. No. 4,326,919 to Hill. This patent shows a control rod spider assembly which includes a plurality of control rods and a spider structure supporting the control rods at their upper ends. The spider structure, in turn, is connected to a control drive mechanism that vertically raises and lowers (referred to as a stepping action) the control rods into and out of the hollow guide thimbles of the fuel assembly. The typical construction of the control rod used in such an arrangement is in the form of an elongated metallic cladding tube having a neutron absorbing material disposed within the tube and with end plugs at opposite ends thereof for sealing the absorber material within the tube.
The spider structure typically includes a plurality of radially extending vanes supported on and circumferentially spaced about a central hub. The vanes are flat metal plates positioned on edge and being connected at their inner ends to the central hub. Cylindrical shaped control rod connecting fingers are mounted to and are supported by the varies, with some of the vanes having only a single connecting finger and other vanes having a spaced pair of connecting fingers associated therewith.
Typically, the upper end plug of each control rod has a threaded outer end which is receivable into a bore in the lower portion of one finger of the spider structure and threadable into a lapped hole formed in the finger at the inner end of the bore. The end plug is then secured or locked therein by a key or pin inserted into the side of the finger and the end plug and then welded therein, as more particularly described in U.S. Pat. No. 4,855,100.
The current design has performed quite well for several years, but in a few instances, control rodlets have been dropped during operation of the reactor. A dropped rodlet is a very undesirable event and has potential for significant safety implications. Root cause evaluations have been completed on these dropped rodlet incidents and it has been determined that the rodlets were dropped because of inadequacies in the anti-rotation features of this design. The anti-rotation features comprise a hole drilled through the finger into the rodlet end plug extension and a pin is installed and welded to prevent the rodlet from unscrewing. There are several disadvantages to this approach. First, the threads in the finger are in the top of a blind hole, which means that any burr chip or nick in the thread may cause the joint not to be preloaded properly and potentially leave the joint susceptible to fatigue and failure. Secondly, assembly is slow because holes for each rodlet must be drilled during assembly. Thirdly, holes must be drilled for each rodlet to a tight tolerance depth so that the pin engages the thin finger wall. If not done properly, the rodlet can turn and become disengaged from the assembly (there have been past issues with dropped rodlets because the hole was drilled too deeply.) Fourthly, the small pin is difficult to handle. Additionally, if the locking weld is not correct for any rodlet, then there is an increased possibility of scrapping an assembly.
Accordingly, a new design is desired that will assure that the threaded joint of the control rod is preloaded as designed during manufacture. Additionally, a new design is desired that will provide assurance that the rodlet will not become disengaged during operation. Furthermore, a new design is desired that will improve manufacturing and reduce cost and assembly time. In addition, a new design is required that will improve quality control inspection and not risk distorting the rodlet flex joint during assembly.